Turbulent boundary layers are responsible for up to 89 % of the skin friction drag of civil aircraft. This shows that in regard to the current social sensitivity towards climate friendly aviation a reduction of tur- bulent skin friction could have a large impact. On the one side greenhouse gas emissions by aviation could be reduced and on the other hand fuel costs could be saved, an important consideration for air- lines. The research on turbulent drag reduction distinguishes between passive and active techniques. Passive techniques have the advantage that no energy input is necessary. Prominent examples are riblets and dimples which have shown turbulent drag reductions up to 10 %. Active techniques reach turbulent drag reduction percentages of up to 45 %, however it is in doubt if the energy savings are larger than the energy required to operate the control technique. One promising active technique which has proven to partially generate a positive energy effect is spanwise wall oscillation. However, researchers have opposing opinions of the mechanism which leads to the drag reduction. As turbulent boundary layers are strongly three-dimensional, the work of this thesis investigates three-dimensional flow fields of turbulent boundary layers subjected to spanwise wall oscillation by means of tomographic PIV. These are used to study the changes in pointwise statistics and coherent structures in order to derive a descriptive model of the drag reducing mechanism.